thanks a lot for your quick explanations and advices.
I wasn't aware that regexp would change my expression internally into something else (and I am actually not sure if I want that). But I kind of see the problem now (as beeing a German).

The German sharp s 'ß' doesn't exist as a capital letter in German writing. Since no German word starts with a sharp s, there is no need for a capital accordance. If used never the less in capital writing, it is written as 'SS'. So at least from a 'German point of view' it makes sense that the 'ss' extension is only implemented for the case-insensitive modifier.